The field of the present invention is needleless hypodermic injection devices. More particularly, the invention relates to needleless hypodermic devices utilizing pressurized gas for injection of medication.
Various needleless hypodermic injection devices have been known in the past. For example, Morrow et al, U.S. Pat. No. 4,790,824 describes a needleless hypodermic injection device having a two-stage gas delivery system and an ampule shroud containing medication which is driven through the skin via gas pressure.
Parsons et al, U.S. Pat. No. 4,680,027 discloses an injection device using a pressurized gas cartridge to drive a piston against the biasing force of a spring. The driven piston works on a syringe causing liquid medication to be ejected with sufficient pressure to penetrate the skin of the patient.
While these and other known injection devices have met with varying degrees of success, their constructions or operating features can prevent effective injection. It has now been discovered that the injection of liquid medication or injectant should be as instantaneous as possible. With gas powered injection devices, the rise time of the gas pressure acting on the piston, and the resulting acceleration at which the piston and injectant are driven is critical. When the gas pressure acting on the piston rises too slowly, the initial medication ejected from the device does not have sufficient pressure or velocity to pass through the skin. In addition, if the "rise time" of the injection sequence is not sufficiently fast, a substantial portion of the medication will be too slowly driven from the device causing a "splash back" condition. Consequently, as a result of "splash back" the patient does not receive the full dosage of medication.
In gas driven injection devices, there are several factors which may effect the efficiency of the device. For example, devices having a long or tortuous gas path will have slower rise times due to flow losses and gas volume compressibility effects. In addition, certain injection devices rely on direct mechanical valve operation by the user of the device to release the gas pressure during the injection sequence. Since the valve operation is done manually in these devices, the effectiveness of the injection can vary widely with the user, due to the speed, activating force and completeness of activating movement employed by different users of the device. More importantly, it has not been previously appreciated that many of these types of devices have relatively large "dead" spaces or volumes of gas trapped behind the piston when the device in the ready to fire condition. These dead volumes substantially hinder injection by slowing the rise time of the gas pressure acting on the piston since substantial time is required for relatively large volumes of gas to flow into the dead volumes to build up an adequate injection pressure.
Gas driven injection devices can also be inadvertently activated if the valve of the device is inadvertently depressed or opened by the user, or if the user should drop the device, etc. This results in wasted injected medication and driving compressed gas.
Gas driven injection devices using compressed gas cartridges can provide a limited number of injections before the cartridge must be replaced with a fresh cartridge. With each injection, some compressed gas is expended thereby decreasing the available supply of compressed gas remaining in the device. After a certain number of injections, the available compressed gas pressure within the device becomes inadequate for proper injection. Consequently, depending on the type of device and the type of injections being provided by the device, it has been necessary for the user of the device to keep track of the number of injections provided by the cartridge in the device, and to replace the cartridge after a maximum specified number of injections. If the maximum number of injections per cartridge is exceeded, the decreased and insufficient gas pressure available can lead to "splash back" as described above.
Accordingly, it is an object of the invention to provide an improved needleless hypodermic injection device.
It is another object of the invention to provide a novel ampule assembly which may be advantageously used with such a needleless injection device.
It is another object of the invention to provide a novel method of subcutaneous or intramuscular injection.
It is yet another object of the invention to provide such a needleless injection device having an interlock to help prevent inadvertent actuation of the device.
It is still another object of the invention to provide such an injection device having a gas pressure indicator to indicate whether the device has sufficient gas pressure for the next injection.